Africa-wide "Great Green Wall" to Halt Sahara's Spread?

China built its famous Great Wall to keep out marauders. Now, millennia later, a "Great Green Wall" may rise in Africa to deter another, equally relentless invader: sand.

The proposed wall of trees would stretch from Senegal to Djibouti as part of a plan to thwart the southward spread of the Sahara, Senegalese officials said earlier this month at the UN's Copenhagen climate conference.

The trees are meant "to stop the advancement of the desert," Senegalese president and project leader Abdoulaye Wade told National Geographic News in Copenhagen.

In many central and West African countries surrounding the Sahara, climate change has slowed rainfall to a trickle, according to the UN's Intergovernmental Panel on Climate Change (IPCC).

Crops have died and soils have eroded—crippling local agriculture. If the trend continues, the UN forecasts that two-thirds of Africa's farmland may be swallowed by Saharan sands by 2025 (explore an interactive Sahara map).

Trees are almost always formidable foes against encroaching deserts, said Patrick Gonzalez of the University of California, Berkeley's Center for Forestry.

That's because stands of trees act as natural windbreaks against sandstorms, and their roots improve soil health—especially by preventing erosion.

But choosing the right tree species to populate the wall will be crucial to the project's success, Gonzalez said via email.

Similar tree-planting efforts by outside agencies have failed, he said, in part because they planted foreign species that soon perished in the harsh desert.

"We Have to Do What We Have to Do"

Nigerian President Olusegun Obasanjo first proposed the idea of a desert-blocking wall in 2005, and it was approved by the African Union in 2007.

All 11 countries that would house the Great Green Wall have pledged to help fund the project.